LONDON (Reuters) - AstraZeneca Plc's (LSE:AZN.L - News) key cholesterol drug Crestor failed to beat Pfizer Inc's (NYSE:PFE - News) Lipitor in a head-to-head clinical study, limiting the protection the British company will get as its U.S. rival prepares for competition from cheap generic copies. Although imaging tests showed patients on Crestor had a lower percentage of artery-clogging plaque than those on Lipitor, the difference in this main goal of the study was not statistically significant, AstraZeneca said on Friday. Crestor did show a significant improvement over Lipitor in a secondary endpoint.